This invention relates to apparatus for correcting zigzag motion of an elongated web, such as an elongated film or a sheet-like material, which is being transported longitudinally.
Consider, for example, a vertical pillow-type packaging machine having installed thereon a sheet roll with an elongated printed sheet wound therearound. Such a sheet is not always uniformly wound in the lateral direction because its thickness may not be uniform in the direction or there were variations in its tension when it was being wound. When such a sheet roll is set on a packaging machine and a sheet is pulled out of it, there may result a zigzag motion of the sheet, and the designs and characters printed on the produced bags may not match between the left-hand and right-hand halves. In order to prevent such occurrences, a mechanism may be used inclusive of a sensor placed along the travel path of the sheet and a correcting means placed on the upstream side of the sensor for shifting the sheet by a closed-loop correction algorithm. Such a mechanism as described, for example, in U.S. Pat. No. 4,049,213 may be effective if the irregularities in the winding of the rolled sheet vary relatively slowly. If the displacements of the sheet in the lateral direction vary significantly, however, there may be situations where a correction has already been effected by the correcting mechanism on the upstream side of the sensor by the time the displacement is detected at a downstream position. In other words, the zigzag motion of the web may become increased, instead of being reduced, such correction based on displacements detected on the downstream side.